Spin-torque magnetoresistive memory element and method of fabricating same

ABSTRACT

A spin-torque magnetoresistive memory element has a high magnetoresistance and low current density. A free magnetic, layer is positioned between first and second spin polarizers. A first tunnel barrier is positioned between the first spin polarizer and the free magnetic layer and a second tunnel barrier is positioned between the second spin polarizer and the free magnetic layer. The magnetoresistance ratio of the second tunnel barrier has a value greater than double the magnetoresistance ratio of the first tunnel barrier.

TECHNICAL FIELD

The exemplary embodiments described herein generally relate tomagnetoresistive random access memory (MRAM) and more particularlyrelate to spin-torque MRAM elements.

BACKGROUND

Magnetoelectronic devices, spin electronic devices, and spintronicdevices are synonymous terms for devices that make use of effectspredominantly caused by electron spin. Magnetoelectronics are used innumerous information devices to provide non-volatile, reliable,radiation resistant, and high-density data storage and retrieval. Thenumerous magnetoelectronics information devices include, but are notlimited to, Magnetoresistive Random Access Memory (MRAM), magneticsensors, and read/write heads for disk drives.

Typically an MRAM includes an array of magnetoresistive memory elements.Each magnetoresistive memory element typically has a structure thatincludes multiple magnetic layers separated by various non-magneticlayers, such as a magnetic tunnel junction (MTJ), and exhibits anelectrical resistance that depends on the magnetic state of the device.Information is stored as directions of magnetization vectors in themagnetic layers. Magnetization vectors in one magnetic layer aremagnetically fixed or pinned, while the magnetization direction ofanother magnetic layer may be free to switch between the same andopposite directions that are called “parallel” and “antiparallel”states, respectively. Corresponding to the parallel and antiparallelmagnetic states, the magnetic memory element has low and high electricalresistance states, respectively. Accordingly, a detection of theresistance allows a magnetoresistive memory element, such as an MTJdevice, to provide information stored in the magnetic memory element.There are two completely different methods used to program the freelayer: field switching and spin-torque switching. In field-switchedMRAM, current carrying lines adjacent to the MTJ bit are used togenerate magnetic fields that act on the free layer. In spin-torqueMRAM, switching is accomplished with a current pulse through the MTJitself. The spin angular momentum carried by the spin-polarizedtunneling current causes reversal of the free layer, with the finalstate (parallel or antiparallel) determined by the polarity of thecurrent pulse. Spin-torque transfer is known to occur in MTJ devices andgiant magnetoresistance devices that are patterned or otherwise arrangedso that the current flows substantially perpendicular to the interfaces,and in simple wire-like structures when the current flows substantiallyperpendicular to a domain wall. Any such structure that exhibitsmagnetoresistance has the potential to be a spin-torque magnetoresistivememory element. The mean current required to switch the magnetic stateof the free layer is called the critical current (Ic). The criticalcurrent density (Jc) is the average critical current per area of the bit(Jc=Ic/A), and the current supplied by the circuit to switch spin-torqueMRAM elements in a memory array is the write current (Iw). Reducing thewrite current Iw is desirable so that a smaller access transistor can beused for each bit cell and a higher density, lower cost memory can beproduced. Lowering Jc is desirable to prevent tunnel barrier damageduring programming.

In order to reduce write current, some spin-torque MRAM elementsincorporate a dual-spin-filter structure, in which the MTJ stackincludes two different spin-polarizing layers, one on each side of thefree layer, to lower Jc by improving spin-torque transfer efficiencythrough increased spin torque on the free layer, resulting in a lowerwrite current. Some dual-spin-filter devices have two tunnel barriersfor providing a lower Jc, and a more symmetrical write current in thecurrent up/down direction, than single tunnel barrier devices.

Dual-spin-filter devices require that the spin-polarizing fixed layerson either side of the free layer have opposite magnetization directions,so that the spin-torque effect from each of the two fixed layers willact together to switch the free layer magnetization into the desireddirection when a current flows either up or down through the device. Oneway to provide such opposed fixed layers is to use a pinned syntheticantiferromagnetic (SAF) fixed region on one side and a single pinnedlayer on the opposite side of the free layer. Another knowndual-spin-filter device includes a three-layer SAF and a two-layer SAFon opposed sides of the free layer. However, a device having suchopposed fixed layers has reduced magnetoresistance ratio (MR) comparedto a single-tunnel-barrier device since one tunnel junction is in theparallel state when the other is in the antiparallel state.

The structure will have a different resistance depending on the stablemagnetic states in which the free magnetic layer has been written. Inorder to achieve a magnetic element which includes a better read signal,or an improved MR, a larger difference between the individualresistances, and thus a larger MR, is desirable.

Accordingly, it is desirable to provide a spin-torque magnetoresistivememory element having a low critical current density and a high MR.Furthermore, other desirable features and characteristics of theexemplary embodiments will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

A spin-torque magnetoresistive memory element is formed on a substratehaving a surface defining a plane. The spin-torque magnetoresistivememory element comprises a first electrode comprising a ferromagneticmaterial formed over the substrate; a second electrode; a free magneticlayer; a first tunnel barrier positioned between the free magnetic layerand the first electrode to form a first tunnel junction having a firstmagnetoresistance ratio and a first resistance-area product; and asecond tunnel barrier positioned between the free magnetic layer and thesecond electrode to form a second tunnel junction having a secondmagnetoresistance ratio and a second resistance-area product, whereinthe first magnetoresistance ratio and the first resistance-area productare one of less than half or more than double the secondmagnetoresistance ratio and the second resistance-area product,respectively.

A method for forming the spin-torque magnetoresistive memory element ona substrate having a surface defining a plane, comprises forming a firstelectrode comprising a ferromagnetic material over the substrate,forming a first tunnel barrier over the first electrode, forming a freemagnetic layer over the first tunnel barrier, thereby forming a firsttunnel junction having a first magnetoresistance ratio and a firstresistance-area product, forming a second tunnel barrier over the freemagnetic layer, and forming a second electrode over the second tunnelbarrier, thereby forming a second tunnel junction having a secondmagnetoresistance ratio and a second resistance-area, wherein the firstmagnetoresistance ratio and the first resistance-area produce are one ofless than half or more than double the second magnetoresistance ratioand the second resistance-area product, respectively.

Another method for forming a spin-torque MRAM element comprising forminga first tunnel barrier; forming a second tunnel barrier; forming a freelayer between the first and second tunnel barriers; forming a firstelectrode on a side of the first tunnel barrier opposed to the freelayer, thereby forming a first tunnel junction having a firstmagnetoresistance ratio and a first resistance-area product; and forminga second electrode on a side of the second tunnel barrier opposed to thefree layer. thereby forming a second tunnel junction having a secondmagnetoresistance ratio and a second resistance-area product, whereinthe first magnetoresistance ratio is more than double the secondmagnetoresistance ratio and the first resistance-area produce is morethan double the second resistance-area product.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with an exemplary embodiment;

FIG. 2 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with another exemplary embodiment;

FIG. 3 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with yet another exemplary embodiment;

FIG. 4 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with still another exemplary embodiment

FIG. 5 is a graph of magnetoresistance versus the resistance/area of atop and a bottom tunnel barrier with the free layer comprising Ta, and atop and a bottom tunnel barrier without the Ta insertion in the freelayer;

FIG. 6 is a graph of magnetoresistance versus the resistance/area of adual tunnel barrier device with a Ta insertion within the free layer;

FIG. 7 is a graph of magnetoresistance versus the resistance/area ofdual tunnel barrier device with a Ru layer within the free layer;

FIG. 8 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with a further exemplary embodiment;

FIG. 9 is a cross section of a spin-torque magnetoresistive memoryelement in accordance with yet a further exemplary embodiment;

FIG. 10 is a flow chart in accordance with an exemplary embodiment of aprocess for fabricating a spin-torque magnetoresistive memory element;

FIG. 11 is a flow chart in accordance with another exemplary embodimentof a process for fabricating a spin-torque magnetoresistive memoryelement;

FIG. 12 is a flow chart in accordance with yet another exemplaryembodiment of a process for fabricating a spin-torque magnetoresistivememory element; and

FIG. 13 is a flow chart in accordance with still another exemplaryembodiment of a process for fabricating a spin-torque magnetoresistivememory element.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

For simplicity and clarity of illustration, the drawing figures depictthe general structure and/or manner of construction of the variousembodiments. Descriptions and details of well-known features andtechniques may be omitted to avoid unnecessarily obscuring otherfeatures. Elements in the drawings figures are not necessarily drawn toscale: the dimensions of some features may be exaggerated relative toother elements to assist improve understanding of the exampleembodiments.

Terms of enumeration such as “first,” “second,” “third,” and the likemay be used for distinguishing between similar elements and notnecessarily for describing a particular spatial or chronological order.These terms, so used, are interchangeable under appropriatecircumstances. The embodiments of the invention described herein are,for example, capable of use in sequences other than those illustrated orotherwise described herein.

The terms “comprise,” “include,” “have” and any variations thereof areused synonymously to denote non-exclusive inclusion. The term“exemplary” is used in the sense of “example,” rather than “ideal.”

In the interest of conciseness, conventional techniques, structures, andprinciples known by those skilled in the art may not be describedherein, including, for example, standard magnetic random access memory(MRAM) process techniques, fundamental principles of magnetism, andbasic operational principles of memory devices.

In general, what is described herein is a spin-torque magnetoresistivememory device structure with a high magnetoresistance ratio and a lowspin-torque critical current density. The structure includes a freelayer positioned between first, and second electrodes, a first tunnelbarrier positioned between the first electrode and the free layerforming a first tunnel junction, and a second tunnel barrier positionedbetween the second electrode and the free layer forming a second tunneljunction. The tunnel barriers may comprise MgO, for example. One or bothof the first and second electrodes may comprise fixed magnetic layerswhich act as spin polarizers that provide polarized tunneling currents.In a first exemplary embodiment in which the first and second electrodescomprise, spin polarizers, the first electrode comprises a ferromagneticalloy with low Fe content and a high B content compared to the secondelectrode, and the first tunnel junction has a lower resistance-areaproduct (RA) compared to the second. In a second exemplary embodiment,the free layer includes a high-Fe interface region in contact with thesecond tunnel barrier. In a third exemplary embodiment, the firstelectrode is a spin polarizer, the second electrode is anon-ferromagnetic material, and the first tunnel junction has a higherRA compared to the second. The free layer may include an optionalhigh-Fe interface region in contact with the first tunnel barrier and itmay include a second high-Fe interface region in contact with the secondtunnel barrier. In a fourth exemplary embodiment, the free layer is acompositionally-modulated structure comprising layers of ferromagneticmaterial, preferably a CoFeB alloy, separated by one or more thinnerlayers including a non-ferromagnetic transition metal, such as Ta, Nb,V, Zr, or Ru. The non-ferromagnetic transition metal lowers themagnetization of the free layer which thereby allows for thicker layersthat typically have better magnetic switching characteristics, and itcan be used to tune the exchange coupling for reduced spin-torquecritical current. The free layer may comprise multiple alternatinglayers of CoFeB and thinner layers comprising one or morenon-ferromagnetic transition metals.

During the course of this description, like numbers are used to identifylike elements according to the different figures that illustrate thevarious exemplary embodiments.

The spin-torque effect is known to those skilled in the art. Briefly, acurrent becomes spin-polarized after the electrons pass through thefirst magnetic layer in a magnetic/non-magnetic/magnetic trilayerstructure, where the first, magnetic layer is substantially more stablethan the second magnetic layer. The higher stability of the first layercompared to the second layer may be determined by one or more of severalfactors including: a larger magnetic moment due to thickness ormagnetization, coupling to an adjacent antiferromagnetic layer, couplingto another ferromagnetic layer as in a SAF structure, or a high magneticanisotropy. The spin-polarized electrons cross the nonmagnetic spacerand then, through conservation of spin angular momentum, exert a spintorque on the second magnetic layer that causes precession of the itsmagnetic moment and switching to a different stable magnetic state ifthe current is in the proper direction. When net current ofspin-polarized electrons moving from the first layer to the second layerexceeds a first critical current value, the second layer will switch itsmagnetic orientation to be parallel to that of the first layer. If abias of the opposite polarity is applied, the net flow of electrons fromthe second layer to the first layer will switch the magnetic orientationof the second layer to be antiparallel to that of the first layer,provided the magnitude of the current is above a second critical currentvalue. Switching in this reverse direction involves a fraction of theelectrons reflecting from the interface between the spacer and the firstmagnetic layer and traveling back across the nonmagnetic spacer tointeracting with the second magnetic layer.

Magnetoresistance is the property of a material to change the value ofits electrical resistance depending on its magnetic state. Typically,for a structure with two ferromagnetic layers separated by a conductiveor tunneling spacer, the resistance is highest when the magnetization ofthe second magnetic layer is antiparallel to that of the first magneticlayer, and lowest when they are parallel. The MR is defined as(R_(H)−R_(L))/R_(L), where R_(L) and R_(H) are the device resistance inthe low and high resistance states, respectively. When the spacer layeris a dielectric tunnel barrier, the tunneling resistance is measured bythe resistance-area product (RA), such that the tunneling resistance Rof a device having an area α, for a tunneling current passingperpendicular to the film plane, is given by RA/α. As used herein, theterm “film” is the equivalent of a thin layer, and the term “film plane”is a plane to the surface of a film or layer.

FIG. 1 is a side sectional view of an MRAM device 100 configured inaccordance with an exemplary embodiment. In practice, an MRAMarchitecture or device will include many MRAM devices 100, typicallyorganized in a matrix of columns and rows. The exemplary MRAM bitstructure (or “stack”) 100 generally includes a free magnetic layer (or“free layer”) 102 separated from a top electrode 104 and a bottomelectrode 106 by tunnel barriers 108 and 110, respectively. Either orboth of the tunnel barriers 108 and 110 may be dielectrics, typicallyoxides such as MgO or AlOx. For the purposes of clarity, somecommonly-used layers have not been illustrated in the drawings,including various protective cap layers, seed layers, and the underlyingsubstrate (which may be a conventional semiconductor substrate or anyother suitable structure). For the exemplary embodiments describedbelow, the bottom electrode 106 is a ferromagnetic polarizer, while thetop electrode 104 may be either a non-ferromagnetic material or aferromagnetic polarizer. Generally, a ferromagnetic polarizer wouldinclude a pinning layer, a pinned magnetic layer, a coupling spacerlayer, and a fixed magnetic layer adjacent to the tunnel barrier (noneof which are shown) as is well known in the industry.

The three layers including the free layer 102, the tunnel barrier 110and bottom electrode 106, forms a first magnetic tunnel junction havinga MR greater than zero and a first RA. The three layers including thefree layer 102, the tunnel barrier 108, and top electrode 104, forms asecond magnetic tunnel junction having a MR equal to, or greater thanzero and a second RA. For the MRAM device 100 to have a good MR, it isdesirable to have the second MR less than half the first MR, and thesecond RA is less than half of the first RA. Most preferably, the secondMR is less than one-fourth the first MR, and the second RA is less thanone-fourth of the first RA.

The difference in the RA of the two tunnel barriers can be adjusted byeither changing the thickness of the tunnel barrier layers 108, 110 orby using different doses of oxidation when forming the dielectrics. TheMR for each junction can be adjusted by using thin layers at the tunnelbarrier interfaces 107, 109 that are either low polarization or highpolarization interfacial layers, as well as through the choice of alloysfor the bottom and top electrodes.

FIG. 2 is a side sectional view of an MRAM device 200 configured inaccordance with an exemplary embodiment in which both electrodes arespin polarizers resulting in what may be referred to as a dualspin-filter MTJ. The exemplary dual spin-filter MTJ bit structure (or“stack”) 200 generally includes a free magnetic layer (or “free layer”)202 separated from a top electrode 204 and a bottom electrode 206 viatunnel barriers 208 and 210, respectively. Both of the layers 208 and210 are dielectrics.

Bottom electrode 206 has a fixed magnetization state that does notchange when the free layer 202 is switched between its two or morestable states. In the practical embodiment, bottom electrode 206 mayinclude a template or seed layer 212 formed on a conductor 222 forfacilitating the formation thereon of a pinning layer 214 made from anantiferromagnetic material, for example, IrMn, PtMn, or FeMn. Thetemplate/seed layer 212 is preferably a non magnetic material, forexample Ta, TaN, Al, Ru, but can also be a magnetic material, forexample NiFe or CoFe. The template/seed layer 212 may include two layersor may be omitted in cases where conductor 222 provides the desiredgrowth characteristics for the subsequent layers. The bottom electrode206 of device 200 includes three ferromagnetic layers 216, 226, and 220,antiferromagnetically coupled, through coupling layers 228 and 213. Thepinning layer 214 determines the orientation of a magnetic moment of thepinned ferromagnetic layer 216 formed thereon. Ferromagnetic layer 226is antiferromagnetically coupled to pinned layer 216, through couplinglayer 228, so that their magnetic moments orient antiparallel in theabsence of an external field and fixed layer 220 isantiferromagnetically coupled to ferromagnetic layer 226, throughcoupling layer 213, so their magnetic moments orient antiparallel in theabsence of an external field. The ferromagnetic layers 216, 226, and 220may be formed from any suitable magnetic material, such as at least oneof the elements Ni, Fe, Co, or their alloys including alloysincorporating additional elements such as B, C, Ta, V, Zr, and others,as well as so-called half-metallic ferromagnets such as NiMnSb, PtMnSb,Fe₃O₄, or CrO₂. In one embodiment, for example, pinned magnetic layer216 and ferromagnetic layer 220 comprises 20-50 of CoFe, fixedferromagnetic layer 220 comprise about 20-30 of CoFeB, and free magneticlayer 202 comprises about 20-35 of CoFeB. Coupling layers 228 and 213are formed from any suitable nonmagnetic material, including at leastone of the elements Ru, Os, Re, Cr, Rh, Cu, Cr, or their combinations.Such synthetic antiferromagnet structures are known to those skilled inthe art and, therefore, their operation will not be described in detailherein. Bottom electrode is chosen to be a SAF with three ferromagneticlayers in device 200 while top electrode 204 is chosen to be a SAF withtwo ferromagnetic layers so that the magnetization direction of fixedlayer 220 and the magnetization direction of top fixed layer 232 will besubstantially antiparallel when processed under typical conditions foran MTJ stack as described below.

In this illustration, arrows are used to indicate the direction of themagnetic moment, or magnetization, for individual layers. Themagnetization directions of the top and bottom fixed layers 220 and 232are typically set using a high-temperature anneal in a strong appliedmagnetic field. During the anneal, the ferromagnetic layers align withthe strong magnetic field. When the field anneal is complete, theantiferromagnetic pinning material, such as that used in pinning layer214, provides an exchange bias to the adjacent ferromagnetic pinnedlayer in the direction of the applied field.

Top electrode 204 includes a non-magnetic layer (“spacer layer,” or“coupling layer”) 230 between two ferromagnetic layers 232 and 234. Themagnetic moments of ferromagnetic layers 232 and 234 areantiferromagnetically coupled through coupling layer 230, so that theirmagnetic moments orient antiparallel in the absence of an externalfield. A top pinning layer 244 can be used to orient the magnetic momentof ferromagnetic layer 234, in the same way that pinning layer 214orients pinned layer 216. After the field anneal, the top pinned layer234 and the bottom pinned layer 216 will be biased in the same directionby the pinning material. Since the SAF that forms the bottom electrodehas one more ferromagnetic layer than does the top electrode, themagnetization of the bottom fixed layer 220 will be set in a directionantiparallel to the magnetization of the top fixed layer 232, providingthe necessary magnetic configuration for additive contributions fromboth fixed layers to the spin torque transferred to the free layer 202.

Top electrode 204 is a synthetic antiferromagnet (SAF) in that itcomprises two ferromagnetic layers separated by a non-magnetic couplinglayer, the thickness of the coupling layer chosen to provide strongantiferromagnetic coupling between the two ferromagnetic layers. Theuseful materials for the layers in top electrode 204 are the same as forbottom electrode 206. In one embodiment, for example, top pinnedmagnetic layer 234 comprises 20-30 of CoFe and ferromagnetic fixed layer232 comprises about 20-30 of CoFeB. It is known in the art, for example,U.S. Pat. No. 7,605,437, that there can be advantages to eliminating thetop pinning layer 244 and instead designing electrode 204 to work as an“unpinned” SAF. The magnetic orientation of the unpinned SAF can be setby designing a magnetic asymmetry into the structure, such as a momentimbalance between the ferromagnetic layers of the SAF 232 and 234.

It is desirable for the magnetic moments of fixed layers 220 and 232 tobe relatively unaffected by spin-transfer torque from free layer 202, sothat only the direction of the free layer 202 changes when a writecurrent is applied. The fixed layers are made stable by the strongcoupling between the layers in each SAF structure and the large magneticvolume of the SAFs compared to the free layer 202. The strong exchangecoupling to the pinning, material contributes additional stability inaddition to defining a reference direction.

First and second conductor 222, 224 are formed from any suitablematerial capable of conducting electricity. For example, conductors 222,224 may be formed from at least one of the elements Al, Cu, Ta, TaNx, Tior their combinations. The various ferromagnetic layers may comprise anysuitable material having the desired ferromagnetic properties asdescribed above. It is advantageous to have the net magnetic couplingexperienced by the free layer to be near zero so that the switchingcharacteristics of the free layer are symmetric. This can be achieved byadjusting the thickness of each ferromagnetic layer in the top andbottom electrodes. There is typically a ferromagnetic coupling between afixed layer and the free layer, due to various mechanisms known in theart. When both top and bottom fixed layers are present, and oriented inopposite directions as shown in device 200, the ferromagnetic interlayercoupling of one fixed layer opposes that of the other, reducing the netcoupling. There is typically antiferromagnetic coupling between thelayers in a patterned magnetic structure due to the poles that form atthe patterned edges of the layers. Since the magnetization of each layerin a SAF structure is opposite to the nearest other ferromagnetic layerin SAF, they have a cancelling effect on each other. In a bottomelectrode comprising a three-layer SAF as shown in device 200, themiddle ferromagnetic layer 226 is typically designed to have a highermagnetic moment than ferromagnetic layers 216 and 220 so that thedipolar field created by layer 226 substantially cancels the dipolarfields created by layers 216 and 220. In an optimized structure, thelayer thicknesses are adjusted so that all the sources of couplingexperienced by the free layer sum to near zero.

In one embodiment, coupling layers 228, 213, 230 comprise Ru having athickness of approximately 8. In an alternate embodiment, some or all ofthe coupling layers may comprise a material, such as Ti or Ta, that doesnot produce any antiparallel coupling between continuous magnetic films,but merely causes exchange decoupling between the magnetic films. Inthis embodiment, the ferromagnetic SAF layers will beantiferromagnetically coupled due to the magnetostatic dipolar fieldsgenerated at the patterned edges of each layer. These alternate couplinglayers will be useful for devices patterned to dimensions less thanapproximately 30 nm because this type of magnetostatic coupling isstronger for smaller patterned shapes.

During fabrication of MRAM structure 200, each succeeding layer (i.e.,layers 222, 212, 214, 216, 228, 226, 213, 220, 210, 202, 208, 232, 230,234, 244, 224) is deposited or otherwise formed in sequence and eachMRAM bit may be defined by selective deposition, photolithographyprocessing, and etching in accordance with any of the variousconventional techniques known in the semiconductor industry. Duringdeposition of the various fixed and free magnet layers, a magnetic fieldmay be provided to set a preferred easy magnetic axis of the layer(i.e., via induced anisotropy). Similarly, a strong magnetic fieldapplied during the post-deposition high-temperature anneal step may beused to induce a preferred easy axis and a preferred pinning directionfor any antiferromagnetically pinned materials.

Free magnetic layer 202 is formed from a ferromagnetic material havingtwo or more stable magnetic states. For example, free magnetic element202 may be formed of various ferromagnetic alloys comprising at leastone of the elements Ni, Fe, and Co. Additional elements are added to thealloys to provide improved magnetic, electrical, or microstructuralproperties. As with conventional MRAM devices, the direction of themagnetization of free magnetic element 202 determines the resistance ofthe element. In practice, for a two-state device, the direction of themagnetization of free magnetic element 202 is either parallel oranti-parallel to the magnetization of a fixed magnetic layer, resultingin a low or high resistance representing a “0” bit state or a “1” bitstate. Furthermore, the free magnetic element 202 may have an in-planemagnetization while the ferromagnetic spin polarizer has out-of-planemagnetization.

Free magnetic layer 202 has a magnetic easy axis that defines a naturalor “default” axis of its magnetization. When MRAM device 200 is in asteady state condition with no current applied from conductor 222 toconductor 224, the magnetization of free magnetic element 202 willnaturally point along its easy axis. MRAM device 200 is suitablyconfigured to establish a particular easy axis direction for freemagnetic element 202. From the perspective of FIG. 2, the easy axis offree magnetic element 202 points either to the right or to the left. Inpractice, MRAM device 200 utilizes anisotropy, such as shape,crystalline, or interface anisotropy, in the free magnetic layer 202 toachieve the orientation of the respective easy axes. It is understood bythose skilled in the art that some materials have a strong perpendicularanisotropy which can be used to make free layers with the two magneticstates lying along a perpendicular easy axis so the two magnetic statesare up and down in FIG. 2. For such devices, one or more perpendicularfixed layer is also used.

In addition to carrying the write current, conductors 222 and 224 alsoserve as the data read conductors for MRAM device 200. In this regard,data in MRAM device 200 can be read in accordance with conventionaltechniques: a small current flows through MRAM device 200 and electrode224, and that current is measured to determine whether the resistance ofMRAM device 200 is relatively high or relatively low. The read currentis much smaller than the current required to switch the free layer byspin-torque in order to avoid disturbs caused by reading the cell.

In practice, MRAM device 200 may employ alternative and/or additionalelements, and one or more of the elements depicted in FIG. 2 may berealized as a composite structure or combination of sub-elements. Thespecific arrangement of layers shown in FIG. 2 merely represents onesuitable embodiment of the invention.

In order to determine a change in state of a magnetic element, amagnetoresistance must be sufficiently high. Three exemplary embodimentsare described herein for providing this high magnetoresistance alongwith a low critical current density (Jc). For a structure with twotunnel junctions, the MR is maximized when one junction dominates theresistance change by having a much larger resistance change than theother junction when the free layer changes state. This is bestaccomplished by having a dominant junction with both a larger. MR and alarger RA than the other junction.

In the present invention, high MR of the dominant tunnel junction isaccomplished by using higher Fe content at the tunnel-barrier interfacesof the dominant junction as compared to the other junction. To make thetunnel junction formed by layers 208, 202, and 232 (FIG. 2) the high-MRjunction, the surfaces in contact with tunnel barrier 208, which are thesurfaces of the adjacent ferromagnetic layers 202 and 232, shouldcomprise a higher Fe content than the surfaces in contact with tunnelbarrier 210. In accordance with one exemplary embodiment, it ispreferred that fixed layer 220, which is the fixed ferromagnetic layeradjacent to tunnel barrier 210, have a low Fe content of less than 20%by atomic composition and a B content greater than 20% by atomiccomposition, and more preferably a low Fe content of approximately 5%and a B content of approximately 25% by atomic composition. The fixedlayer 232, adjacent to tunnel barrier 208 may have an Fe content greaterthan 20% by atomic composition and a B content of between 14% and 20% byatomic composition.

Referring to FIG. 3 and in accordance with another exemplary embodiment,the device 300, includes a deposition of a small amount of iron (Fe)between the tunnel barrier 208 and the free magnetic layer 202. The thinFe interface deposition may form a continuous atomic layer of Fe or maymix with the underlying free ferromagnetic alloy in the final annealedstructure, resulting in a high-Fe interface region 302 adjacent totunnel barrier 208. It should be noted that all components of thisexemplary embodiment as illustrated in FIG. 3 that are similar tocomponents of the exemplary embodiment of FIG. 2 are designated withlike numbers. The top electrode 204 shown in FIG. 3 as an unpinned SAFas described previously, and the bottom electrode 206 is a pinned SAFwith two ferromagnetic layers. The amount of the Fe deposition may be inthe range of 0.5 Å to 5 Å, but preferably is in the range of 1.5 Å-3 Å,expressed in equivalent continuous film thickness (see U.S. Pat. No.7,098,495 assigned to the assignee of the present application regardinghigh polarization insertion layers). By adding a small amount of pure Feat the interface between tunnel barrier 208 and the free layer 202, toform high-Fe interface region 302, one can obtain higher MR values evenwhen free layer 202 is predominantly comprised of low-Fe, high-B CoFeBalloy. Whether the high-Fe interface region includes a continuous atomiclayer of Fe, a discontinuous layer of Fe, or an interfacial layer ofhigh-Fe alloy, it does result in at least an atomic layer of material atthe surface of the free layer composed mainly of Fe atoms. That is, theinterface region 302 will be at least 50% Fe by atomic percentage.Adding Fe under tunnel barrier 208 also improves the growth of 208 andincreases RA for a given tunnel barrier process. The deposited Fe alsoimproves the growth of the (001) crystallographically-oriented MgO layerupon it. The preferred device design has a low RA and low MR for thetunnel junction formed by tunnel barrier 210 through use of a low-RAtunnel barrier process and low-Fe, high-B alloys for fixed layer 220 andfree layer 202, combined with a high RA and high MR for the tunneljunction formed by tunnel barrier 208 by forming a high-Fe interfaceregion 302 below tunnel barrier 208 combined with a high-RA tunnelbarrier process. The Fe-rich surface may provide higher perpendicularinterface anisotropy energy than for the case where the top tunnelbarrier 208 is grown on a free layer, for example, of a CoFeB alloy,without the Fe deposition. The perpendicular interface anisotropy isdesirable since it lowers the spin-torque switching critical current Icby offsetting some of the thin-film demagnetization anisotropy thatresults in a strong in-plane anisotropy. The interfacial perpendicularanisotropy lowers Ic by making it easier for the free layer moment toprocess out of plane as needed in the switching process.

In another aspect of this invention, it has been found that insertingcertain materials into the free layer can increase the MR of the toptunnel junction formed by tunnel barrier 208 and sometimes decrease theMR of the bottom tunnel junction formed by tunnel barrier 210.

In still another exemplary embodiment is device 400 as shown in FIG. 4,the free layer 202 comprises a thin layer 402 comprising either Ta or Rupositioned between a first portion 404 and a second portion 406. The Tainsertion layer deposition is chosen to be of a thickness that does notform a continuous Ta layer, which would break the exchange between theadjacent layers, but rather mixes with the other free layer materials orforms a layer that is not continuous so that the adjacent ferromagneticlayers 404 and 406 are directly exchange coupled to each other and theentire structure 202 acts as a single ferromagnetic free layer. Thetypical thickness of Ta deposited to achieve this effect is less than3.5 Å, and preferably in the range between 1 Å and 3 Å. Other similarmaterials that form alloys with Co, Fe, or Ni may yield similar results,for example: V, Zr, Ti, Nb, Mo, W, Hf, Mn, or Cr. The Ru insertion layerthickness is chosen to be of a thickness that results in a continuouslayer for antiferromagnetic coupling but may have gaps whenferromagnetic coupling is desired, thus being as thin as 2 Angstrom,with little or no alloying with the adjacent ferromagnetic layers 404and 406. For Ru and similar materials, the ferromagnetic layers 404 and406 are coupled through this nonmagnetic layer by the well-knownoscillatory exchange coupling effect and are considerednon-ferromagnetic coupling layers. The coupling strength associated withsuch non-ferromagnetic coupling layers is controlled by the layerthickness, preferably between 2 Angstrom and 30 Angstrom, and most oftenbetween 5 Angstrom and 15 Angstrom. Other similar materials that mayyield similar results include: Rh, Os, Cu, Cr, Pd, Pt, or Ir. It shouldbe noted that all components of this exemplary embodiment as illustratedin FIG. 4 that are similar to components of the exemplary embodiment ofFIG. 3 are designated with like numbers.

The graph 500 of FIG. 5 shows experimental data of magnetoresistance(MR) versus resistance-area product (RA) for single junctions of a toptunnel barrier 502 with the Ta insertion 402 in the free layer 202 (data502) compared with a top tunnel barrier without the Ta insertion 402(data 504), and a bottom tunnel barrier with the Ta insertion 402 in thefree layer 202 (data 508) compared with a bottom tunnel barrier withoutthe Ta insertion 402 (data 506). In the case where the top tunnelbarrier forms the dominant magnetic tunnel junction of a dual tunnelbarrier device, adding the Ta insertion 402 can be expected to increasethe MR of the device since it would enhance the MR of the dominantjunction and reduce the MR of the other junction. The symbols aremeasured data points for MTJ stacks made with MgO tunnel barriers andCoFeB ferromagnetic layers having a high-Fe interface region 302 for thetop tunnel-barrier stacks. The various RA values were obtained byvarying the oxidation dose of the tunnel barrier. Note that, to improveMR in a dual-spin-filter structure, the MR of the bottom junction can befurther lowered by using a low-Fe alloy for the bottom fixed layer 220.

The graph 600 of FIG. 6 shows magnetoresistance (MR) versus RA data forthe structure of FIG. 4 with the Ta insertion 402 in the free layer 202(data 602) compared with the structure of FIG. 3 without the Tainsertion 402 (data 604). It is seen that the Ta insertion 402 providesan improvement in MR averaging approximately 10 percentage points overthe resistance-area product range shown.

The graph 700 of FIG. 7 shows magnetoresistance (MR) versus RA for thestructure of FIG. 4 with a Ru layer 402 in the free layer 202 (data 702)compared with the same type of data for the structure of FIG. 3 withouta Ru layer 402 (data 704), after a field anneal at 300 degrees C. It isseen that the Ru layer 702 provides an improvement in MR of 30 to 50percentage points over the resistance-area product range shown.

FIG. 8 is a side sectional view of a free layer 802 configured inaccordance with another exemplary embodiment which may be used in lieuof the free layer 202 of FIGS. 2, 3, and 4. Two insertion layers 812 aredeposited between ferromagnetic material layers 814, 816, 820, whereinthe material used for the insertions and the amount of the material arechosen as described for insertion layer 402 in FIG. 4. Fe is depositedon the top ferromagnetic layer 820 to form high-Fe interface region 302,and an optional high-Fe interface region 818 is formed on the bottomtunnel barrier 210 by depositing Fe on tunnel barrier 210 beforedepositing ferromagnetic layer 814. Though only two insertion layers areshown, additional such layers could be formed within the free layer 802.In one preferred embodiment, both insertion layers 812 comprise Tadepositions including between 0.5 Å and 3.5 Å ofcontinuous-film-equivalent, material, although the layers are notcontinuous and do not break the exchange coupling between theferromagnetic layers 814, 816, 820. In the final structure, the regionsof Ta deposition 812 are thin films of Ta-rich ferromagnetic alloy ordiscontinuous regions of Ta lying in the plane between the ferromagneticmaterials. The additional Ta reduces the magnetization of the compositefree layer material, enabling thicker free layers for a given desiredmagnetic moment, resulting in better magnetic properties than thethinner layers of the same CoFeB alloy without the Ta insertions. In asecond preferred embodiment, one of the insertion layers 812 comprisesTa as described above and another comprises Ru with a thickness chosenas described above for layer 402 of FIG. 4. A Ru insertion layer with anoptimized coupling strength can provide reduced switching current (asdescribed in US Patent Publication 2009/0096042) while combining withthe desirable reduced magnetization and perpendicular anisotropyprovided by the other insertion layer and Fe-rich surface layerdescribed in this invention. Note that ferromagnetic layers 814, 816,and 820 do not need to be the same thickness or material, but cancontain different materials and compositions and thicknesses as desiredfor optimum performance. Ferromagnetic layers 814, 816, and 820 arepreferably thin-film depositions of CoFeB alloy with less than 10% Feand more than 14% B, and most preferably about 5% Fe and 25% B by atomicconcentration, each deposition in the thickness range of 5 Å to 20 Å asneeded to obtain the desired total magnetic moment for the free layer802.

As described previously for high-Fe interface region 302 in FIG. 3, thehigh-Fe interface regions 302 and 818 in FIG. 8 may include a continuousatomic layer of Fe, a discontinuous layer of Fe, or an interfacial layerof high-Fe alloy, resulting in at least an atomic layer of material atthe surface of the free layer composed mainly of Fe atoms. That is, thehigh-Fe interface regions 302 and 818 will be at least 50% Fe by atomicpercentage. The Fe-rich interface regions may provide a higherperpendicular interface anisotropy energy than for the case where thetop tunnel barrier 208 is grown on a surface without the Fe deposition.

FIG. 9 is a side sectional view of an MRAM device 900 configured inaccordance with an exemplary embodiment in which the bottom electrode206 (FIG. 3) includes a ferromagnetic fixed layer 220 in contact withthe bottom tunnel barrier 210 and the top electrode 204 of FIG. 3 isreplaced by a non-ferromagnetic material 930. This double-tunnel-barrierstructure is not a dual spin filter since it has a ferromagnetic layerpolarizing the tunneling electrons on one side only. However, it isfound that the top tunnel junction formed by tunnel barrier 208 enablesa significant reduction in the critical current Ic required to switchthe free layer, even though top electrode 930 is not ferromagnetic. Theimprovement may arise from magnetic heating of the free layer by theelectrons tunneling through tunnel barrier 208 and from perpendicularinterface anisotropy resulting from the interface between the surface ofthe free layer 202 and the top tunnel barrier 208 as described above fordevice 802 in FIG. 8. Since top electrode 930 is not ferromagnetic, MRfor the junction formed by tunnel barrier 208 is zero and this junctionwill be the non-dominant junction in the double-tunnel-barrier device.For maximum MR of device 900, the resistance of the tunnel junctionformed by tunnel barrier 208 should be much less than that of thedominant junction formed by tunnel barrier 210. The RA of barrier 210should be at least two times higher than RA of barrier 208, and mostpreferably it should be more than four times higher. The double-barrierstructure 900 is useful because it provides many of the benefits of thedual spin filter structures 200, 300, and 400 but with a simpler andthinner top electrode, making the material stack much easier to patterninto devices. However, the choice of materials for the non-ferromagnetictop electrode 930 and the process for making the top tunnel barrier 208are critical to proper functionality of the devices as described below.A further benefit of double-barrier structure 900 is that the top tunneljunction, having low RA and no magnetoresistance as described above;does not produce a resistance change opposite to the dominant bottomtunnel junction, resulting in a higher MR for the device 900 compared todual spin filter devices 200, 300, and 400.

The free layer 202 includes a ferromagnetic layer 201 and surface layer302 in device 900 is most preferably the free layer 802 as shown in FIG.8 and described above. In the preferred embodiment, the ferromagneticlayers are a CoFeB alloy. Optionally and most preferably, a high-Feinterface region 818 is formed on tunnel barrier 210 by depositing Fe ontunnel barrier 210. Preferably a thin Fe layer is deposited on top ofthe final ferromagnetic layer to form the high-Fe interface region 302below the top tunnel barrier 208. Since the tunnel junction formed bytunnel barrier 208 has little or no MR due to the non-ferromagnetic topelectrode 930, this Fe deposition does not affect MR as it does for thedual spin filter structures, but has been found to promotes the growthof high quality MgO for tunnel barrier 208 as well as promotingperpendicular magnetic anisotropy at the interface between the freelayer 202 and the tunnel barrier 208. The MTJ stack of 900 using thefree layer 802 provides all benefits of the free layer 802 and thedouble tunnel barrier device 900.

It is desirable that the interface between tunnel barrier 208 and thetop electrode 930 is of a very high quality so that the tunnel junctionformed by tunnel barrier 208 will be free from defects, shorting, andexcessive spatial variation of the tunneling current. To form a highquality interface, the choice material for top electrode 930 isimportant as is the material under tunnel barrier 208. The material incontact with tunnel barrier 208 must have properties that allow for asharp interface with the tunnel barrier dielectric, typically MgO. SinceFe and Co form such sharp interfaces, those materials and alloys basedon those materials can be used for non-ferromagnetic top electrode 930if they are very thin, preferably less than or equal to 15 Angstrom ofdeposited ferromagnetic alloy, and a layer of Ta or similar material isdeposited on them to suppress their ferromagnetism to the point wherethe resulting layer is not ferromagnetic within the operatingtemperature range of the device. Examples of such Fe and Co alloysinclude Fe, Co, CoFe, and alloys incorporating B, C, Ta, Ti, V, Nb, Zr,W, Hf, Cr, Mo, and Mn. Examples of layers to deposit on these materialsto suppress their ferromagnetism include Ta, Ti, V, Nb, Zr, W, Hf, Cr,Ru, Mo, and Mn. Alternatively a non-ferromagnetic material that forms asharp interface with the dielectric may be deposited on tunnel barrier208, and may optionally be followed by one of the material combinationsdescribed above. The benefit of using a non-ferromagnetic layer first isto eliminate MR and any magnetic coupling to the free layer that wouldbe associated with any residual ferromagnetic material at the interface.The benefit of also including one of the Fe or Co alloys over thenon-ferromagnetic layer is to provide an amorphous layer that is veryresistant to interdiffusion between the MTJ stack and materials from thetop contact 224. Examples of such top electrodes 930 include: Ru, Ru/Ta,CoFeB (<15 Å)/Ta, and Ru/CoFeB (<15 Å)/Ta.

As described above with regards to device 300 in FIG. 3, free layer 802in FIG. 8, and device 900 in FIG. 9, an Fe-rich surface at the interfacewith MgO may provide a higher perpendicular interface anisotropy energythan for the case where the MgO is in contact with a typical CoFeB alloyused for the free layer, without the Fe deposition 302 applied. For MTJdevices having in-plane magnetic easy axes, the perpendicular interfaceanisotropy lowers the spin-torque switching critical current Ic byoffsetting some of the thin-film demagnetization anisotropy that resultsin a strong in-plane anisotropy. The interfacial perpendicularanisotropy lowers Ic by making it easier for the free layer moment toprecess out of plane as needed in the switching process. However, if theinterface anisotropy is strong enough, and the moment of a ferromagneticlayer low enough, it is possible for the perpendicular interfaceanisotropy to overcome the in-plane thin-film demagnetizationanisotropy, resulting in a film with a perpendicular easy axis. Anadditional embodiment of the present invention employs the free layer802 of FIG. 8 or the free layer 202 of FIG. 4, in a double tunnelbarrier structure or dual spin filter structure, designed with strongperpendicular anisotropy and a magnetic moment low enough to have aperpendicular easy axis. In this case, the two stable states of the freelayer will be with the magnetization vector pointed perpendicular to theplan up toward tunnel barrier 208 or down toward tunnel barrier 210.Similar layers, with or without an antiferromagnetic pinning layer, canbe used to form all or part of the bottom and top electrodes with aperpendicular magnetization.

FIG. 10 is a flow chart that illustrates an exemplary embodiment of aprocess 1000 for fabricating an MRAM device having a highmagnetoresistance and a low critical current density. It should beappreciated that process 1000 may include any number of additional oralternative tasks, the tasks shown in FIG. 10 need not be performed inthe illustrated order, and process 1000 may be incorporated into a morecomprehensive procedure or process having additional functionality notdescribed in detail herein. Moreover, one or more of the tasks shown inFIG. 10 could be omitted from an embodiment of the process 1000 as longas the intended overall functionality remains intact.

The method 1000 for forming a spin-torque magnetoresistive elementcomprises: forming 1002 a first electrode; forming 1004 a first tunnelbarrier over the first electrode; forming 1006 a free magnetic layerover the first tunnel barrier to form first magnetic tunnel junction,wherein the first magnetic tunnel junction has a MR greater than zeroand a first RA, forming 1008 a second tunnel barrier over the freelayer, and forming 1010 a second electrode over the second tunnelbarrier to form a second magnetic tunnel junction, wherein the secondmagnetic tunnel junction has a second MR equal to or greater than zeroand a second RA, and wherein the second MR is less than half the firstMR, and the second RA is less than half the first RA. Forming the layerstypically involves thin-film deposition processes known in the art,including but not limited to physical vapor deposition techniques suchas ion beam sputtering and magnetron sputtering. Forming thin insulatinglayers, such as the tunnel barrier layers, may involve physical vapordeposition from an oxide target, such as by radio-frequency (RF)sputtering, or by deposition of a thin metallic film followed by anoxidation step, such as oxygen plasma oxidation, oxygen radicaloxidation, or natural oxidation by exposure to a low-pressure oxygenenvironment. Devices are typically defined by photolithography andetching steps known in the fields of integrated circuit manufacturingand magnetoresistive sensor manufacturing.

Referring to FIG. 11, a method for forming a dual tunnel barrierstructure including an insertion layer and an Fe deposition includesforming 1102 a first electrode over a substrate, the first electrodeincluding a first fixed magnetic layer, forming 1104 a first tunnelbarrier on the first fixed magnetic layer, forming 1106 a free layer onthe first tunnel barrier to create a first magnetic tunnel junctionhaving a first magnetoresistance and a first resistance-area product,wherein forming the free layer includes the steps of depositing a firstferromagnetic portion, depositing an amount of a non-ferromagneticmaterial corresponding to less than 4 angstroms in thickness, depositinga second ferromagnetic portion, and depositing an amount of ironcorresponding to less than or equal to 5 Angstroms in thickness, thenon-ferromagnetic material comprising at least one of Ta, Nb, Hf, Zr,Ti, W, Cr, and Mn. A second tunnel barrier is formed 1108 on the freelayer, and a second electrode is formed 1110 on the second tunnelbarrier, the second electrode including a second fixed magnetic layer incontact with the second tunnel barrier to create a second magnetictunnel junction having a second magnetoresistance and a secondresistance-area product, wherein the magnitude of the secondmagnetoresistance is at least twice that of the first magnetoresistanceand the magnitude of the second resistance-area product is at leasttwice that of the first resistance-area product.

Referring to FIG. 12, a method for forming a dual tunnel barrierstructure including a coupling insertion layer and an Fe depositionincludes forming 1202 a first electrode over a substrate, the firstelectrode including a first fixed magnetic layer, forming 1204 a firsttunnel barrier on the first fixed magnetic layer, forming 1206 a freelayer on the first tunnel barrier to create a first magnetic tunneljunction having a first magnetoresistance and a first resistance-areaproduct, wherein forming the free layer includes the steps of depositinga first ferromagnetic portion, depositing an amount of anon-ferromagnetic coupling material corresponding to a thickness ofbetween 2 Angstrom and 30 Angstrom, depositing a second ferromagneticportion, and depositing an amount of iron corresponding to a thicknessof less than or equal to 5 Angstroms, the non-ferromagnetic couplingmaterial comprising at least one of Ru, Rb, Ir, Pt, Pd, Cu, Cr, and Os.A second tunnel barrier is formed 1208 on the free layer, and a secondelectrode is formed 1210 on the second tunnel barrier, the secondelectrode including a second fixed magnetic layer in contact with thesecond tunnel barrier to create a second magnetic tunnel junction havinga second magnetoresistance and a second resistance-area product, whereinthe magnitude of the second magnetoresistance is at least twice that ofthe first magnetoresistance and the magnitude of the secondresistance-area product is at least twice that of the firstresistance-area product.

Referring to FIG. 13, a method for forming a double tunnel barrierstructure including a non-ferromagnetic layer in contact with the secondtunnel barrier and an optional Fe deposition includes forming 1302 afirst electrode over a substrate, the first electrode including a firstfixed magnetic layer, forming 1304 a first tunnel barrier on the firstfixed magnetic layer, forming 1306 a free layer on the first tunnelbarrier to create a first magnetic tunnel junction having a firstmagnetoresistance and a first resistance-area product, wherein formingthe free layer includes the steps of depositing a first ferromagneticportion, depositing an amount of a non-ferromagnetic material,depositing a second ferromagnetic portion, and optionally depositing anamount of iron corresponding to a thickness of less, than or equal to 5angstroms, the non-ferromagnetic material comprising an, amountresulting in magnetic exchange coupling between the first and second,ferromagnetic portions. A second tunnel barrier is formed 1308 on thefree layer, and a second electrode is formed 1310 on the second tunnelbarrier, the second electrode including a non-ferromagnetic layer incontact with the second tunnel barrier to create a second magnetictunnel junction having no magnetoresistance and a second resistance-areaproduct, wherein the magnitude of the second resistance-area product isless than half that of the first resistance-area product.

In summary, a magnetic element and fabricating method thereof isdisclosed in which the MR is improved based on the inclusion of Fe atthe high-magnetoresistance tunnel barrier, an Fe layer between the freelayer and the top tunnel barrier, and a portion comprising anon-ferromagnetic transition metal within the free layer, such as aninterlayer including Ta or an Ru coupling interlayer between first andsecond portions of the free layer.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention, it beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set forth in the appendedclaims.

1. A spin-torque magnetoresistive memory element formed on a substratehaving a surface defining a plane, the spin-torque magnetoresistivememory element comprising: a first electrode comprising a ferromagneticmaterial formed over the substrate; a second electrode; a free magneticlayer; a first tunnel barrier positioned between the free magnetic layerand the first electrode to form a first tunnel junction having a firstmagnetoresistance ratio and a first resistance-area product; and asecond tunnel barrier positioned between the free magnetic layer and thesecond electrode to form a second tunnel junction having a secondmagnetoresistance ratio and a second resistance-area product, whereinthe first magnetoresistance ratio and the first resistance-area productare one of less than half or more than double the secondmagnetoresistance ratio and the second resistance-area product,respectively.
 2. The spin-torque magnetoresistive memory element ofclaim 1 wherein the first electrode comprises a first ferromagnetic spinpolarizer having a first magnetization vector, and the second electrodecomprises a second ferromagnetic spin polarizer having a secondmagnetization vector with a direction substantially opposite to thefirst magnetization vector.
 3. The spin-torque magnetoresistive memoryelement of claim 1 wherein the first electrode comprises a ferromagneticspin polarizer and the second electrode comprises a non-ferromagneticmaterial, wherein the second tunnel junction has a tunnelingresistance-area product less than one-half the tunneling resistance-areaproduct of the first tunnel junction; and the second tunnel junction haszero tunneling magnetoresistance.
 4. The spin-torque magnetoresistivememory element of claim 2 wherein the first electrode comprises aferromagnetic material with an Fe content higher than the secondelectrode.
 5. The spin-torque magnetoresistive memory element of claim 4wherein the first and second electrodes comprise CoFeB and the secondelectrode comprises less than 20% Fe by atomic composition and greaterthan 14% B by atomic composition.
 6. The spin-torque magnetoresistivememory element of claim 1 wherein the free magnetic layer comprises: afirst portion comprising a CoFeB layer; and a second portion consistingof a material between the first portion and the first tunnel barrierhaving a thickness less than first portion, the material comprising Fegreater than 50 percent by atomic composition.
 7. The spin-torquemagnetoresistive memory element of claim 6 wherein the free magneticlayer further comprises a third portion comprising a material betweenthe first portion and the second tunnel barrier having a thickness lessthan first portion, the material comprising Fe greater than 50 percentby atomic composition.
 8. The spin-torque magnetoresistive memoryelement of claim 1 wherein the free magnetic layer comprises a CoFeBalloy with atomic composition of Fe less than 20% and B greater than 19%by atomic composition.
 9. The spin-torque magnetoresistive memoryelement of claim 1 wherein the free magnetic layer comprises: a firstportion consisting of a first ferromagnetic material; a second portionconsisting of a second ferromagnetic material; and a third portioncomprising at least one of Ta, Nb, V, Hf, Zr, Ti, W, Mo, Cr and Mn in anamount corresponding to less than 4 Angstrom deposited between andferromagnetically coupling the first and second portions.
 10. Thespin-torque magnetoresistive memory element of claim 9 wherein the firstferromagnetic material and the second ferromagnetic material comprise analloy of B and at least one of Co and Fe.
 11. The spin-torquemagnetoresistive memory element of claim 9 wherein the firstferromagnetic material and the second ferromagnetic material compriseCoFeB alloys with atomic composition of Fe less than 20% and B greaterthan 19% by atomic composition; and the free layer comprises a thirdportion adjacent the first tunnel barrier having a thickness less thaneach of the first portion and the second portion, the third portioncomprising Fe greater than 50 percent by atomic composition.
 12. Thespin-torque magnetoresistive memory element of claim 1 wherein the freemagnetic layer comprises: a first portion consisting of a firstferromagnetic material comprising CoFeB alloys with atomic compositionof Fe less than 20% and B greater than 19% by atomic composition; asecond portion consisting of a second ferromagnetic material comprisingCoFeB alloys with atomic composition of Fe less than 20% and B greaterthan 19% by atomic composition; a third portion positioned between thefirst and second portions and comprising at least one of Ru, Rh, Ir, Pt,Pd, Cu, Cr, and Os in an amount corresponding to between 2 Angstrom and30 Angstrom of deposited material; and a layer positioned between thefirst tunnel barrier and the first portion comprising Fe greater than 50percent by atomic composition, wherein the first portion and the secondportion are coupled together either ferromagnetically oranti-ferromagnetically by the third portion.
 13. The spin-torquemagnetoresistive memory element of claim 9 wherein the free magneticlayer further comprises: a fourth portion consisting of a thirdferromagnetic material; a fifth portion comprising a secondnon-ferromagnetic material comprising at least one of Ta, Nb, V, Hf, Zr,Ti, W, Mo, Cr and Mn in an amount corresponding to less than 4 Angstromof deposited material between and ferromagnetically coupling the secondportion and the fourth portion.
 14. The spin-torque magnetoresistivememory element of claim 9 wherein the free magnetic layer furthercomprises: a fourth portion consisting of a third ferromagneticmaterial; a fifth portion comprising at least one of Ru, Rh, Ir, Pt, Pd,Cu, Cr, and Os in an amount corresponding to between 2 Angstrom and 30Angstrom of depoited material between the second portion and the fourthportion; wherein the second portion and the fourth portion are coupledtogether either, ferromagnetically or anti-ferromagnetically by thefifth portion.
 15. The spin-torque magnetoresistive memory element ofclaim 13 wherein the first ferromagnetic material and the secondferromagnetic material are alloys of B and at least one of Co and Fe.16. The spin-torque magnetoresistive memory element of claim 13 whereinthe first ferromagnetic material, the second ferromagnetic material andthird ferromagnetic material comprise CoFeB alloys with atomiccomposition of Fe less than 20% and B greater than 19% by atomiccomposition; and the free layer further comprising a layer adjacent tothe first tunnel barrier having a thickness less than each of the firstportion, the second portion, and the fourth portion, the layercomprising Fe greater than 50 percent by atomic composition.
 17. Thespin-torque magnetoresistive memory element of claim 16 wherein the freemagnetic layer further comprises a layer adjacent to the second tunnelbarrier having a thickness less than each of the first, portion, thesecond portion, and the fourth portion, the layer comprising Fe greaterthan 50 percent by atomic composition.
 18. The spin-torquemagnetoresistive memory element of claim 1 wherein the first and secondtunnel barriers comprise MgO.
 19. The spin-torque magnetoresistivememory element of claim 3 wherein the non-ferromagnetic material of thesecond electrode comprises one or more of Ru, Os, Rh, Ir, Pd, and Pt.20. The spin-torque magnetoresistive memory element of claim 3 furthercomprising: an amorphous metal layer positioned between thenon-ferromagnetic material and the top conductor.
 21. The spin-torquemagnetoresistive memory element of claim 20 wherein the amorphous metallayer comprises an alloy including at least one of Co and Fe and atleast one of B, C, Si, Ta, V, Nb, and Zr.
 22. The spin-torquemagnetoresistive memory element of claim 1 wherein the easy magneticaxis of the free layer and the easy magnetic axis of the first electrodeare perpendicular to the plane.
 23. The spin-torque magnetoresistivememory element of claim 9 wherein a first perpendicular anisotropy isassociated with the interface between the free layer and the firsttunnel barrier and a second perpendicular anisotropy is associated withthe interface between the free layer and the second tunnel barrier,wherein the total interface anisotropy induces a perpendicular easy axisanisotropy in the free layer resulting in two stable magnetic stateshaving a magnetization vector directed perpendicular to the plane eithertoward or away from the first electrode.
 24. The spin-torquemagnetoresistive memory element of claim 12 wherein a firstperpendicular anisotropy is associated with the interface between thefree layer and the first tunnel barrier and a second perpendicularanisotropy is associated with the interface between the free layer andthe second tunnel barrier, wherein the total interface anisotropyinduces a perpendicular easy axis anisotropy in the free layer resultingin two stable magnetic states having a magnetization vector directedperpendicular to the plane either toward or away from the firstelectrode
 25. The spin-torque magnetoresistive memory element of claim12 wherein the free layer has a first magnetic vector and theferromagnetic spin polarizer has a second magnetic vector perpendicularto the first magnetic vector, and wherein the free layer has in-planemagnetization while the ferromagnetic spin polarizer has out-of-planemagnetization.
 26. A method for forming a spin-torque magnetoresistivememory element on a substrate having a surface defining a plane,comprising: forming a first electrode comprising a ferromagneticmaterial over the substrate; forming a first tunnel barrier over thefirst electrode; forming a free magnetic layer over the first tunnelbarrier, thereby forming a first tunnel junction having a firstmagnetoresistance ratio and a first resistance-area product; forming asecond tunnel barrier over the free magnetic layer; forming a secondelectrode over the second tunnel barrier, thereby forming a secondtunnel junction having a second magnetoresistance ratio and a secondresistance-area, wherein the first magnetoresistance ratio and the firstresistance-area produce are one of less than half or more than doublethe second magnetoresistance ratio and the second resistance-areaproduct, respectively.
 27. The method of claim 26 wherein forming thefirst electrode comprises forming a first ferromagnetic spin polarizerhaving a first magnetization vector, and forming the second electrodecomprises forming a second ferromagnetic spin polarizer having a secondmagnetization vector with a direction substantially opposite to thefirst magnetization vector.
 28. The method of claim 26 wherein formingthe first electrode comprises forming a ferromagnetic spin polarizer,and forming the second electrode comprises forming a non-ferromagneticmaterial, wherein the second tunnel junction has a tunnelingresistance-area product less than one-half the tunneling resistance-areaproduct of the first tunnel junction.
 29. The method of claim 27 whereinforming the first electrode comprises forming the first electrode with aferromagnetic material having an Fe content higher than the secondelectrode.
 30. The method of claim 29 wherein forming the first andsecond electrodes comprise forming with CoFeB, and forming the secondelectrode comprises forming with less than 20% Fe by atomic compositionand greater than 14% B by atomic composition.
 31. The method of claim 26wherein forming the free magnetic layer comprises: forming a firstportion comprising CoFeB layer; and depositing a second portionconsisting of Fe between the first portion and the first tunnel barrierin an amount corresponding to a thickness less than or equal to fiveAngstroms of Fe material.
 32. The method of claim 31 further comprising:depositing a third portion consisting of Fe between the first portionand the second tunnel barrier in an amount corresponding to a thicknessless than or equal to five Angstroms of Fe material.
 33. The method ofclaim 26 wherein forming the free magnetic layer comprises forming aCoFeB alloy with atomic composition of Fe less than 20% and B greaterthan 19% by atomic composition.
 34. The method of claim 26 whereinforming the free magnetic layer comprises: forming a first portionconsisting of a first ferromagnetic material; depositing anon-ferromagnetic material in an amount corresponding to a filmthickness of less than 4 Angstrom and comprising at least one of Ta, Nb,Hf, Zr, Ti, W, Cr, and Mn; forming a second portion consisting of asecond ferromagnetic material; and depositing an amount of ironcorresponding to a film thickness of less than or equal to 5 Angstrom.35. The method of claim 34 wherein forming the first ferromagneticmaterial and the second ferromagnetic material comprise forming an alloyof B at least one of Co and Fe.
 36. The method of claim 34 whereinforming the first ferromagnetic material and the second ferromagneticmaterial comprises forming CoFeB alloys with atomic composition of Feless than 20% and B greater than 19% by atomic composition.
 37. Themethod of claim 26 wherein forming the free magnetic layer comprises:forming a first portion consisting of a first ferromagnetic materialcomprising CoFeB alloys with atomic composition of Fe less than 20% andB greater than 19% by atomic composition; depositing a non-ferromagneticmaterial in an amount corresponding to a film thickness between 2Angstrom and 30 Angstrom and comprising at least one of Ru, Rh, Ir, Pt,Pd, Cu, Cr, and Os; forming a second portion consisting of a secondferromagnetic material comprising CoFeB alloys with atomic compositionof Fe less than 20% and B greater than 19% by atomic composition;depositing an amount of Fe in an amount corresponding to a filmthickness of less than or equal to 5 Angstrom of deposited material;wherein the first portion and the second portion are coupled togethereither ferromagnetically or anti-ferromagnetically by thenon-ferromagnetic material.
 38. The method of claim 34 wherein the freemagnetic layer further comprises: depositing a second non-ferromagneticmaterial on the second ferromagnetic material in an amount correspondingto a film thickness between 2 Angstrom and 30 Angstrom and comprising atleast one of Ru, Rh, Ir, Pt, Pd, Cu, Cr, and Os; forming a third portionconsisting of a third ferromagnetic material on the secondnon-ferromagnetic material; wherein the second portion and the thirdportion are coupled together either ferromagnetically oranti-ferromagnetically by the second non-ferromagnetic material.
 39. Themethod of claim 34 wherein the free magnetic layer further comprises:depositing a second non-ferromagnetic material on the secondferromagnetic material in an amount corresponding to less than 4Angstroms of deposited material and comprising at least one of Ta, Nb,Hf, Zr, Ti, W, Cr, and Mn; forming a third portion consisting of a thirdferromagnetic material; wherein the second portion and the third portionare ferromagnetically coupled.
 40. The method of claim 39 whereinforming the first ferromagnetic material and the second ferromagneticmaterial and the third ferromagnetic material comprises forming alloysof B and at least one of Co and Fe.
 41. The method of claim 40 whereinforming alloys of B and at least one of Co and Fe comprises formingCoFeB alloys with Fe less than 20% and B greater than 19% by atomiccomposition.
 42. The method of claim 26 wherein forming the first andsecond tunnel barriers comprise forming MgO.
 43. The method of claim 28wherein forming the non-ferromagnetic material of the second electrodecomprises forming one or more of Ru, Os, Rh, Ir, Pd, and Pt.
 44. Themethod of claim 28 further comprising: forming an amorphous metal layerbetween the non-ferromagnetic material and the top conductor.
 45. Themethod of claim 44 wherein forming the amorphous metal layer comprisesforming an alloy including at least one of Co and Fe and at least one ofB, C, Si, Ta, V, Nb, and Zr.
 46. The method of claim 26 wherein formingthe easy magnetic axis of the free layer and the easy magnetic axis ofthe first electrode comprises forming the easy magnetic axis of the freelayer and the easy magnetic axis of the first electrode perpendicular tothe plane.
 47. The method of claim 34 wherein a first perpendicularanisotropy is associated with the interface between the free layer andthe first tunnel barrier and a second perpendicular anisotropy isassociated with the interface between the free layer and the secondtunnel barrier, wherein the total interface anisotropy induces aperpendicular easy axis anisotropy in the free layer resulting in twostable magnetic states having a magnetization vector directedperpendicular to the plane either toward or away from the firstelectrode.
 48. The method of claim 37 wherein a first perpendicularanisotropy is associated with the interface between the free layer andthe first tunnel barrier and a second perpendicular anisotropy isassociated with the interface between the free layer and the secondtunnel barrier, wherein the total interface anisotropy induces aperpendicular easy axis anisotropy in the free layer resulting in twostable magnetic states having a magnetization vector directedperpendicular to the plane either toward or away from the firstelectrode.
 49. The method of claim 37 wherein forming the free layercomprises forming the free layer having a first magnetic vector andforming the ferromagnetic spin polarizer has a second magnetic vectorperpendicular to the first magnetic vector, and wherein the free layerhas in-plane magnetization while the ferromagnetic spin polarizer hasout-of-plane magnetization.
 50. The method of claim 31 wherein formingthe easy magnetic axis of the free layer and the easy magnetic axis ofthe first electrode comprises forming the easy magnetic axis of the freelayer and the easy magnetic axis of the first electrode perpendicular tothe plane.
 51. A method for forming a spin-torque MRAM element,comprising: forming a first tunnel barrier; forming a second tunnelbarrier; forming a free layer between the first and second tunnelbarriers; forming a first electrode on a side of the first tunnelbarrier opposed to the free layer, thereby forming a first tunneljunction having a first magnetoresistance ratio and a firstresistance-area product; and forming a second electrode on a side of thesecond tunnel barrier opposed to the free layer thereby forming a secondtunnel junction having a second magnetoresistance ratio and a secondresistance-area product, wherein the first magnetoresistance ratio ismore than double the second magnetoresistance ratio and the firstresistance-area produce is more than double the second resistance-areaproduct.